This application is a 371 of PCT/de97/01854, filed Aug. 26, 1997.
1. Field of the Invention
The present invention relates to a method for obtaining a temperature-coefficient-compensated output signal in an optical sensor for measuring a periodically fluctuating electrical and/or magnetic field strength.
2. Description of the Related Art
Optical sensors of the cited type are known, in particular in the form of current measurement sensors for measuring the current strength of an alternating current conducted in a conductor, which produces in the immediate environment of the conductor an electromagnetic field with an electrical and magnetic field strength that fluctuates periodically corresponding to the current strength of the alternating current. The sensor measures, for example according to the Faraday effect, the periodically fluctuating magnetic field strength, according to which the current strength can be inferred.
Voltage measurement sensors operate in a similar manner for the measurement of an alternating voltage applied to a conductor, which voltage produces in the immediate environment of the conductor an electrical field with an electrical field strength that fluctuates periodically corresponding to the alternating voltage. This sensor measures, for example according to the Pockels effect, the periodically fluctuating electrical field strength, according to which the alternating voltage can be inferred.
The respective field strength is measured in that light that can be influenced by the field strength is sent through the periodically fluctuating field, and from this light that has been sent through and influenced two intensity signals are produced that are separate from one another and that comprise intensities containing intensity portions that fluctuate in chronologically periodic fashion in phase opposition to one another dependent on the field strength of the periodically fluctuating field.
The influencing of the light by the field can be based on various known physical effects, for example the cited Faraday or Pockels effect. As an example, the Faraday effect, which is frequently used in current measurement sensors, is explained in more detail, in which linearly polarized light from a polarizer is influenced by the magnetic field strength in such a way that the polarization plane of the polarized light is rotated, dependent on the field strength, in relation to the polarization plane determined by the polarizer, or in correspondingly periodically fluctuating fashion in the case of the periodically fluctuating field strength. The linearly polarized light which has been influenced by the magnetic field strength can be supplied to an analyzer, which forms two polarized light portions that are perpendicular to one another from this light, whose intensities relative to one another depend, in phase opposition, on the polarization state of the field-strength-influenced light in relation to the polarization plane determined by the polarizer, and which supplement each other to form an overall intensity that is independent of the angular position of the polarization plane of the field-strength-influenced light. In this case, these light portions form the two intensity signals that indicate the periodically fluctuating field strength [sic].
Such intensity signals can be obtained in a similar manner in voltage measurement sensors which for example exploit the Pockels effect.
With the aid of the variable derived from the two intensity signals, which corresponds to the quotient of the difference of the intensities of the two intensity signals and the sum of these intensities and that includes the direct portion and the periodically fluctuating alternating portion, to which a root-mean-square value can be allocated, the output signal of such a sensor, indicating the measured field strength and therewith the current strength or voltage, is obtained.
This output signal, in particular a periodically fluctuating portion contained therein, often exhibits a temperature drift whose cause is grounded in the physical effect on which the measurement is based, and/or in disturbances such as for example mechanical stresses and/or a linear double refraction.
In order to remove this problem, for example a temperature coefficient compensation method is proposed in relation to a magneto-optical sensor based on the Faraday effect, in which the two intensity signals are obtained in the manner described above with the aid of linearly polarized light from a polarizer, for example a laser diode, and an optical analyzer. In this known temperature compensation method, it is necessary to adjust the polarizer and analyzer very precisely to one another with respect to their angular setting, i.e., in such a way that the polarization plane determined by the polarizer and the polarization plane determined by the analyzer stand as precisely as possible at an angle of 45xc2x0 to one another.
The present invention provides a new type of method for obtaining a temperature-coefficient-compensated output signal, having the advantage that in a sensor of the cited type, if the two intensity signals are obtained with the aid of a polarizer and analyzer, larger angular misadjustments are permissible between the polarizer and the analyzer in comparison to the known temperature compensation method.
The method, in which the quantity is derived from the two intensity signals, which quantity corresponds to a quotient of a difference of the intensities of the two intensity signals and the sum of these intensities, is known as xe2x88x92/+ intensity norming.
The basic idea of the invention underlying the inventive method permits, but is not limited to, this intensity norming, but rather can also be applied analogously to sensors in which what is known as AC/DC intensity norming is present, i.e., in sensors.
The invention likewise provides a new type of method for obtaining a temperature-coefficient-compensated output signal, with the advantage that in a sensor of the named type, if the two intensity signals are obtained with the aid of a polarizer and analyzer, greater angular misadjustments are permissible between the polarizer and the analyzer in comparison to the known temperature compensation method.
In the inventive method, angular misadjustments between the polarizer and the analyzer of more than 5xc2x0 advantageously adversely affect the function of the inventive method only inessentially in the sensor itself. The reduction of sensitivity is low and is proportional to the cosine of the faulty angle between the polarization plane of the polarizer and that of the analyzer. This advantageously simplifies the design and manufacture of the sensor in which the inventive method is applied.
However, the inventive method is not limited to sensors that operate with polarizer and analyzer, but rather can generally be applied also in other sensors, and in this way extends the technology.
The function to be used in the inventive method and predetermined by the calibration measurement can be approximated by a polynomial of a predeterminable degree and/or can be stored in approximate form in a lookup table.
A preferred arrangement for the execution of a method for obtaining a temperature-coefficient-compensated output signal in an optical sensor for measuring a periodically fluctuating electrical and/or magnetic field strength, in which sensor
light that can be influenced by the field strength is sent through the periodically fluctuating field strength,
from this light that has been sent through and influenced, two intensity signals are produced that are separate from one another and that comprise intensities containing intensity portions that fluctuate in chronologically periodic fashion in phase opposition to one another dependent on the periodically fluctuating field strength, and
a quantity is derived from the two intensity signals, which quantity corresponds to a quotient of a difference of the intensities of the two intensity signals and the sum of these intensities, whereby
the quantity comprises a direct portion and a periodically fluctuating alternating portion with a root-mean-square value, in that the temperature-coefficient-compensated output signal is obtained by formation of a quotient of the alternating portion of the derived quantity and a function, determined by a calibration measurement, of the direct portion and of a root-mean-square value of the alternating portion of the derived quantity, this being executed by
a means for obtaining the direct portion of the derived quantity,
a means for obtaining the alternating portion of the derived quantity and formation of the root-mean-square value of this portion, and
a means for forming the quotient of the direct portion of the derived quantity and the function) of the direct portion and of the root-mean-square value of the alternating portion of the derived quantity. A preferred arrangement for the execution of a method for obtaining a temperature-coefficient-compensated output signal in an optical sensor for measuring a periodically fluctuating electrical and/or magnetic field strength, in which sensor
light is sent through the periodically fluctuating field strength,
from this light that has been sent through, two intensity signals are produced comprising intensities that contain alternating portions that fluctuate periodically in phase opposition to one another dependent on the periodically fluctuating field strength and that contain a respective direct portion, and
a respective normed quantity is derived from each of the two intensity signals, which normed quantity corresponds to a quotient of the alternating portion and the direct portion of the intensity of this intensity signal, in that from the two derived quantities
a first quotient is derived from the sum and a difference of the two normed quantities,
a periodically fluctuating second quotient is derived from the twofold product and the difference of the two normed quantities, and
the temperature-coefficient-compensated output signal is obtained
by formation of a quotient of the periodically fluctuating second quotient and a function, determined by a calibration measurement, of the first quotient and of a root-mean-square value of the periodically fluctuating second quotient, this being executed by
a means for formation of the first quotient from the sum and the difference of the two derived normed quantities,
a means for formation of the periodically fluctuating second quotient of the twofold product and the difference of the two derived normed quantities and of the root-mean-square value of the second quotient,
a means for formation of the quotient of the formed periodically fluctuating second quotient and the function, determined by the calibration measurement, of the formed first quotient and of a root-mean-square value of the periodically fluctuating second quotient.